Silicoaluminophosphates (SAPOs) have been used as absorbents and catalysts. As catalysts, SAPOs have been used in processes such as fluid catalytic cracking, hydrocracking, isomerization, oligomerization, the conversion of alcohols or ethers, and the alkylation of aromatics. To be useful as a catalysts, SAPO molecular sieves have to be activated. Methods of activating SAPOs are known.
U.S. Pat. No. 4,681,864 to Edwards et al. discuss the use of SAPO-37 molecular sieve as a commercial cracking catalyst. It is disclosed that activated SAPO-37 molecular sieve has poor stability. However, stability can be improved by using a particular activation process. According to the process, organic template is removed from the core structure of the sieve just prior to contacting with feed to be cracked. The process calls for subjecting the sieve to a temperature of 400-800.degree. C. within the catalytic cracking unit.
U.S. Pat. No. 5,185,310 to Degnan et al. discloses another method of activating silicoaluminophosphate molecular sieve compositions. The method calls for contacting a crystalline silicoaluminophosphate with gel alumina and water, and thereafter heating the mixture to at least 425.degree. C. The heating process is first carried out in the presence of an oxygen depleted gas, and then in the presence of an oxidizing gas. The object of the heating process is to enhance the acid activity of the catalyst. The acid activity is enhanced as a result of the intimate contact between the alumina and the sieve.
Briend et al., J.Phys. Chem. 1995, 99, 8270-8276, teach that SAPO-34 loses its crystallinity when the template has been removed from the sieve and the de-templated, activated sieve has been exposed to air. Data is presented, however, which suggest that over at least the short term, crystallinity loss is reversible. Even over a period of a couple years, the data suggest that crystallinity loss is reversible when certain templates are used.
As seen from the disclosure herein, we have found that SAPO molecular sieves will exhibit a shortened catalytic life when the catalytic sites are activated and exposed to a moisture-containing environment. This loss of catalytic life is likely not reversible, and can occur over a very short period of time. In essence, this loss of catalytic life is due to a loss in the number of acid catalytic sites. It is desirable, therefore, to obtain a SAPO molecular sieve that can be activated with little to no loss in acid catalyst sites, and exhibit a relatively long catalytic life. Such a molecular sieve would be highly desirable for use in a wide variety of catalytic compositions, which could be used in a variety of catalytic processes.